The present invention relates to methods and apparatus for bending tubes and more particularly concerns improved apparatus and methods of draw bending.
A common form of bending tubes, particularly automobile exhaust pipes, is the press bending method exemplified by U.S. Pat. Nos. to Lance, 3,808,856 and Inda 3,426,562 wherein a ram die presses the tube against a pair of movable but restrained wing dies. Press bending is useful for bending large numbers of identical tubes but is less satisfactory when it is desired to bend only a relatively small number of tubes of any given configuration. This is so because of the large amount of time and skill embodied in adjusting the machine for individual bend configurations and the relatively great amount of setup time required.
Compression and draw bending may involve less setup time but each has certain disadvantages. In one type of compression bending a tube is clamped to a rotary bend die by a clamp die and another section of the tube is pressed against the bend die by a pressure die. The bend and clamp dies are rotated together with the tube clamped therebetween to wrap the tube around the bend die, concomitantly moving the tube axially. For bends over 15.degree. to 20.degree. of arc, compression bending causes wrinkling or buckling of the tube material on the inside of the bend, a phenomenon which requires difficult, costly and unsatisfactory arrangements to avoid. Thus, a die collar may be employed to gather material on the inside of the bend. This is costly and may interfere with fluid flow through the completed tube.
In some forms of rotary draw bending, the bending apparatus may be similar to that employed in the compression bending so that the tube is clamped between the clamp die and the rotary bend die and the two are rotated together to bend the tube around the bend die. However, as distinguished from the compression bending in which no axial tension is applied to the tube, the draw bending arrangement provides some means to exert axial tension on the tube as it is bent around the bend die thereby effecting a substantial axial elongation of the tube as it is bent. This axial elongation, particularly on the inner side of the bend, overcomes the wrinkling or buckling problem of the compression bending, but at the cost of other disadvantages. For the axial elongation, a sufficient restraint must be placed upon the tube by a pressure die so that the material of the tube can be stressed past its yield point. This is achieved by exerting sufficient pressure on the tube by a pressure die, and concomitantly by the clamp die which presses a forward portion of the tube against the bend die. In order to achieve adequate pressure on the tube between the clamp and bend die for draw bending, the clamp die must press against the tube over a significant length of the tube. Commonly, the clamp die has a length parallel to the extent of the tube in the order of three times the tube diameter. If the clamp die is much smaller than this, the tube is likely to slip relative to the clamp die or, in the alternative, such great force must be exerted by the clamp upon the tube that the tube is often unacceptably deformed. Because large clamp dies are required for draw bending, it is not possible in this type of bending to form tube bends close together. Two successive bends cannot be any closer to each other than the length of the clamp die.
Some types of draw bending employ a mandril within the tube, which itself introduces still other problems. The mandril may operate to exert a force that restrains axial motion of the tube as the clamp and bend die rotate with the clamp tube about the bend die axis. In many cases, this restraint is so great that the pressure die pressing the tube against the bend die with the mandril in the tube is actually driven forward in the direction of the tube axis as the tube is pulled around the bend die. This is done in order to decrease the axial restraint exerted by the friction of the mandril within the tube although sufficient friction remains for drawing of the tube. This forward motion of the pressure due continues in such an operation throughout the entire bending motion.
U.S. Pat. Nos. to H. Monroe 3,145,287 and to W. P. Hill 3,145,756, are exemplary of apparatus for performing this type of rotary draw bending wherein a pressure die, also termed a wiper die, is driven forwardly with the tube to preclude excessive stretching or elongation of the tube due to the friction of the mandril and pressure of the wiper die.
Thus, in draw bending employing a mandril, the tube is stretched as it is bent around the bend die, being firmly grasped by a relatively long clamp die and pressed against the bend die at a forward portion, and being frictionally retained by a combination of pressure die, bend die and internal friction of the mandril at a rearward portion.
The use of the mandril has drawbacks. The mandril is costly. Its operation is often difficult in the presence of internal weld scars on the tubing. Internal lubrication of the tube is required and the operation is slow and expensive.
Accordingly, it is an object of the present invention to perform tube bending with a minimum of the abovementioned disadvantages.